1. Field of the Invention
The present invention relates to a projection display apparatus which projects an image formed by liquid crystal panels onto a screen to provide an enlarged version of the image. More particularly, the present invention relates to a projection display apparatus which comprises an imaging optical system made up of a plurality of reflectors.
2. Description of the Related Art
As one of projection display apparatuses for projecting an image formed by an image forming element to provide an enlarged version of the image, a projection display apparatus is known to comprise an imaging optical system (reflection imaging optical system) which is made up of a plurality of reflectors. FIG. 1 illustrates an example of a reflection imaging optical system. The illustrated reflection imaging optical system is made up of first reflector 51 to fourth reflector 54. Light emitted from a light source, not shown, is led to image forming element 55 through an illumination optical system, also not shown. Image forming element 55 modulates the light incident thereon based on an image signal to form an image (image light). The image light which is formed by image forming element 55 impinges on and reflected by a reflective surface of first reflector 51, and then impinges on a reflective surface of second reflector 52. Subsequently, the image light which is reflected by second reflector 52 is reflected by third reflector 53 and fourth reflector 54 in sequence, and is finally projected toward a screen, not shown.
As illustrated in FIG. 1, each reflector 51-54 is disposed at a position higher than the preceding reflector. Also, each reflector 51-54 is laid out to diagonally and upwardly direct image light toward the next reflector. This is intended to avoid interference of the image light with respective reflectors 51-54. In the reflection imaging optical system having the foregoing configuration, a predetermined reflector is reciprocally moved to adjust the spacings between this reflector and reflectors preceding and subsequent thereto, thereby adjusting the focus (see JP-A-2004-144914). In the illustrated reflection imaging optical system, third reflector 53 has a focus adjusting function. For adjusting the focus, third reflector 53 is moved along reference axis B parallel with the normal line A of image forming element 55 to adjust the spacing between third reflector 53 and fourth reflector 54. Since the focal length extends as third reflector 53 is moved closer to fourth reflector 54, a larger image can be projected by moving the projection display apparatus further away from the screen.
As described above, for projecting a larger focused image, the focal length must be extended by bringing a reflector having a focus adjusting function closer to the next reflector. However, the closer that a reflector, having the focus adjusting function, is brought to the next reflector, the greater is the downward movement of the light beam which is folded back by the next reflector. A more specific description will be given below with reference again to the exemplary reflection imaging optical system illustrated in FIG. 1. As third reflector 53 is brought closer to fourth reflector 54 to extend the focal length, a light beam which is folded back by fourth reflector 54 to stream downward. This results in a smaller clearance between the light beam folded back by fourth reflector 54 and third reflector 53. Eventually, the light beam folded back by fourth reflector 54 interferes with third reflector 53. Accordingly, the amount that third reflector 53 moves must be restricted so that the spacing between both reflectors 53, 54 will be wide enough to avoid the interference when third reflector 53 approaches closest to fourth reflector 54.